Pulmonary artery constriction in adult cats (4.0 mm I.D. clip) results initially in marked depression of myocardial function with improved function 6 weeks after banding and normal function by 24 weeks. The effect of exercise on function of hypertrophied hearts will be determined by subjecting 6 week banded cats to daily treadmill exercise. Twenty-four weeks after banding right ventricular (RV) hypertrophy and length-tension relations, maximum dF/dt, time to peak force, "maximal force index" and force-velocity relations in isolated RV papillary muscles will be measured. To determine the effect of exercise on the response of the normal heart to sustained loads, exercise conditioned cats will be banded and RV hypertrophy and muscle mechanics examined 6 weeks after banding. To determine the effect of hypertrophy and varying states of myocardial function on passive stiffness, stress-strain relations of muscles for normal cats and cats banded for 2-3, 6 and 24 weeks will be examined. The effect of increasing concentrations of acetylstrophanthidin (AS) on passive stiffness will also be determined in these muscles. To determine whether the concentration-inotropic effect relations of AS are effected by the state of the myocardium, muscle mechanics of normal and banded cats will be examined at increasing concentrations of AS. In our banded kitten model, in which the initial phase of depressed myocardial function appears to be avoided, the effect of duration of increased afterload on myocardial function will be examined at various periods up to 24 months after banding. Also, the effect of duration of banding on the rate and extent of regression of hypertrophy after removal of the band will be determined in kittens banded for 6, 12 and 24 months.